1. Field of the Invention
The present invention relates to a fiber-optic signal processing system and, more particularly, to two types of fiber-optic loop elements which can be combined in various configurations to form wideband fiber-optic signal processing systems capable of processing radio frequency (RF) signals, each fiber-optic loop element including a monomode optical fiber formed in a loop by way of a fiber-optic coupling device and including one or more reflection mechanisms, such as a Bragg grating, to provide greater flexibility than known systems in controlling the frequency response of the fiber-optic signal processing system.
2. Description of the Prior Art
The use of fiber-optic systems for wideband radio frequency (RF) signal transmission provides several advantages over conventional RF transmission systems including wider bandwidth, less weight and smaller size. Because of the limited utility of known fiber-optic signal processing systems, fiber-optic RF transmission signals are known to be converted to the electrical domain for various signal processing functions, including filtering and equalization. Unfortunately, because of the limited bandwidth capability of the electronic components used for such electronic signal processing, the bandwidth of the fiber-optic system is limited.
Fiber-optic signal processing methods are known; however, as discussed above, their utility is rather limited. Such fiber-optic signal processing systems normally include a monomode fiber formed in various configurations, as generally described in "Fiber-optic Lattice Signal Processing" by B. Moslehi, J. W. Goodwin, M. Tur, and H. J. Shaw, Proceedings of the IEEE, Vol. 72, No. 7, July 1984, pp. 909-930, hereby incorporated by reference. As discussed in detail in "Fiber-optic Lattice Signal Processing", such lattice filters are known to be configured as either recirculating (feed backward) or non-recirculating (feed forward). An important aspect of the lattice filters is the ability to cascade such filters to create various frequency response characteristics of the filter.
As discussed in U.S. Pat. No. 4,768,850, the frequency response of such lattice filters can be controlled somewhat by controlling the delays in order to manipulate the poles and zeroes of the transfer function to create filters having different frequency response characteristics. However, control of the delays provides only limited flexibility in creating various frequency response characteristics.
Fiber-optic signal processing systems are known to be based upon discrete time techniques and, in particular, are based upon time delay lines and weighted tapping configurations. A time delay line involves adding a predetermined length of fiber to the system in order to create a delay. For example, a fiber about 1 meter long with a refractive index of about 1.5 will have a propagation delay of about 5 nanoseconds. Examples of systems which include time delay lines are disclosed in U.S. Pat. Nos. 4,676,585, 4,934,777 and 5,367,586.
As mentioned above, weighted tap delays are also known to be used in fiber-optic signal processing systems. A weighted tap delay is configured such that the output signal is available at several different points, the distance between which provide a delay time equal to the period of the fundamental frequency of the input signal. As such, the output signal is normally a constant multiple of the input signal. An example of such a system is disclosed in U.S. Pat. No. 4,557,552.